Wastewater treatment plants (WWTPs) may spread Legionella via the air and cause pneumonia if people inhale them. Based on two recent studies on Legionella, RIVM advises that WWTPs with an increased risk should test for the presence of Legionella and take appropriate prevention and control measures.
In 2017 and 2018 two wastewater treatment plants were identified as the probable source of infection for a number of patients with Legionnaires' disease in the Netherlands. This disease is caused by inhalation of the Legionella bacterium.
WWTPs may have specific conditions which promote the growth of Legionella. This is particularly true when water in the treatment basin is warm, and air is blown in. During the treatment process, small droplets of water (aerosols) containing Legionella bacteria may arise and can spread through the air into the environment. A previous study showed that there are approximately 80 WWTPs in the Netherlands with an increased risk of growth and spread of Legionella. In total, there are 776 WWTPs in the Netherlands.
Spread via the air
RIVM used a model to calculate how aerosols with Legionella spread through the air from WWTPs. With this model, RIVM found that patients reported between 2013 and 2018 were more exposed than average to aerosols from this type of installations. This indicates that some of these people may have become infected with the Legionella bacteria from WWTPs. Based on this study alone, the researchers cannot determine which installations may have spread the Legionella bacteria.
Possible control measures
In addition, RIVM has described which measures can prevent the spread of Legionella in WWTPs. RIVM has also investigated the effectiveness of several control measures. Examples of control measures include covering the water basins or filtering air from water basins and disinfect the air with ultraviolet (UV) radiation. Most of these control measures are effective in preventing the spread of Legionella, but the effect of a measure will differ between installations. WWTP’s vary in type and size so that the effect of a measure will differ accordingly. It is therefore important to investigate the risk of growth and spread of Legionella per installation so that each installation can take appropriate measures.
Legionella in the Netherlands
The number of reported patients with pneumonia caused by Legionella in the Netherlands has increased in recent years. In 2013, RIVM received approximately 300 notifications of patients with Legionnaires’ disease, and in 2018 there were approximately 600 patients notified. Well-known sources of Legionella include hot tubs and wet cooling towers. For most patients who become infected in the Netherlands, it remains unclear what the source of the infection was. These studies indicate the importance of including WWTPs in the investigation into the possible source of infection of patients with Legionnaires' disease.